In recent years, in the technical field of circuits and semiconductor devices for driving a motor in a home electric appliance or an industrial facility, a method, by which rectified voltage from commercial power supply or direct-current voltage corresponding to it is applied to a motor directly through an inverter in order to drive the motor, comes into wider use. This method is mainly used for the purpose of achieving high efficiency and miniaturization of the motor. Driving a motor at a high voltage decreases consumed current and prevents a loss, caused by internal resistance or the like, from increasing. In addition, it is possible to make a wiring diameter of motor winding small, which contributes to miniaturization of the motor.
Because driving a brushless motor requires an inverter unit, it is desired that an inexpensive inverter unit is supplied in the field of home electric appliances. For this reason, in an inverter driving unit for the brushless motor, an inexpensive 120-degree energizing method is used. This method permits a simple circuit configuration, and is capable of achieving relatively high motor-efficiency.
As regards a motor driving circuit by means of the 120-degree energizing method, a motor is driven in the following manner: a magnetic-pole position of a motor rotor is detected by a magnetic-pole detector; on/off state of each switching element of an inverter unit is controlled in response to the time at which a magnetic pole of the rotor matches with that of a stator. The magnetic-pole position of the rotor is detected using a Hall generator to which Hall effect is generally applied, or using a Hall IC that has an amplifier embedded in a Hall element. As regards this detected signal, 120 degrees out of 180 degrees, which are expressed in electrical angle, is logically switched on for energizing. That is to say, the remaining 60 degrees perform operation of switching off inverter output. Because of it, current immediately after motor current i has been switched on/off shows a current waveform with extremely large variations (di/dt). This di/dt vibrates motor winding, which causes electromagnetic sound to be emitted outside. A frequency of this electromagnetic sound is proportional to a motor rotational frequency and the number of motor poles. Because the frequency ranges from several Hz to several hundred Hz in a rotation range of actual motor use, the sound becomes audible noise.
In addition, if a motor current waveform includes a large amount of harmonic content, motor torque generally tends to cause ripples. Because the motor torque is a product of motor-specific inductive voltage and motor current, ripples of the motor torque depend on the motor current waveform to a large extent. The torque ripples vibrate the motor itself, which causes a base to which the motor is mounted to vibrate. As a result, a noise is generated.
As a method for reducing the noise, there is a method in which motor driving current is made sinusoidal by means of so called PWM (Pulse Width Modulation) control. Specifically, sinusoidal signal is obtained by detecting magnetic flux of a magnetic pole of a motor stator using a Hall element. This sinusoidal signal is compared with a carrier wave signal, which is an output signal of a carrier wave generator, by a comparator to obtain a PWM signal. On-off control of the inverter unit by the PWM signal keeps motor current in a sinusoidal state. However, if advanced operation is required, there is a problem in that a system becomes more complicated and more expensive than the system using the 120-degree energizing method.
The present invention has been devised by taking the above-mentioned problem into account, and provides a motor driving circuit that has a relatively simple circuit and that generates a low noise.
A motor driving circuit or a driving method comprises the steps of: generating a plurality of instruction signals in response to a motor rotational speed; and selecting one of the plurality of instruction signals according to a magnetic-pole position of a motor, to generate modulated waves. Controlling a power converter, which is used to supply the motor with driving power, by means of the pulse width modulation according to the modulated waves reduces a noise caused by the motor. As a plurality of instruction signals, a direct-current signal that keeps a level proportional to the rotational speed in response to the speed is used. In this case, the modulated waves have a waveform with a level that corresponds to the direct-current level (for example, a stepped waveform). An inverter or the like is used as a power converter. The inverter converts direct-current power to alternating-current power by on-off control of semiconductor switching element.
In the present invention, a low-noise motor driving circuit is integrated into a single semiconductor chip monolithically. The monolithic semiconductor integrated circuit can be built into an enclosure of the motor together with a magnetic-pole position detector. In this connection, the motor driving circuit according to the present invention may be mounted outside the motor enclosure, or may be accommodated inside a resin case to modularize it. Additionally, the motor may be driven in a manner that estimates a magnetic-pole position without using the magnetic-pole position detector, or the like; what is called, in a sensor-less manner.